1. Field of the Invention
The present invention relates to a plate-shaped compression mold for producing sheet laminates made of synthetic resins and having a metal coating on at least one surface thereof. In addition, the invention relates to a process for producing a pressing plate of a plate-shaped compression mold, and a process for producing sheet laminates, in particular printed circuit boards with copper coatings, wherein a corresponding compression mold is employed.
2. Discussion of Background Information
Technical laminates are flat elements, produced by stacking and thermal compression molding of substrates, which are impregnated with synthetic resin, for example, phenolic resin. Paper, fiber mats, such as glass fiber mats, and the like can be used as substrates. The curing process of the synthetic resin at high temperature can take place in several steps. Particularly, partially cured or pre-cured plates are prepared, provided with coatings or the like and/or stacked, and are completely cured in plate-shaped compression molds, for example between pressing plates, under the action of compressive forces and heat, whereby so-called multilayer boards can be produced.
Printed circuit boards, which are used for producing electronic devices and similar assemblies, are synthetic resin laminated elements, which have, at least on one side, a metal coating, in particular a copper coating. These circuit boards are produced by placing copper foil, usually having a thickness of from about from 5 .mu.m to about 35 .mu.m, on the flat surface(s) of the pre-cured laminate, clamping the resulting structure between pressing plates with extremely high surface quality, and heating same under pressure to a temperature of between 150.degree. C. and 450.degree. C. such that complete curing of the synthetic resin takes place.
Pressing plates for making technical laminates generally have a surface area of as much as 3.0 m.sup.2 or more and a thickness of about 0.8 mm to about 3.0 mm with the least possible variation in thickness and extremely high surface quality. Because of the chemical stress and the atmospheric attack, these pressing plates must have high corrosion resistance and a high degree of hardness as well a high wear resistance. To meet these requirements, martensitic chromium steels are usually used for pressing plates, with the plates being tempered according to special hardening processes. Precipitation hardenable iron-base alloys are also used as pressing plate materials.
A great disadvantage of these pressing plate materials is their comparatively low thermal coefficient of linear expansion of about 10.0.times.10.sup.-6 /K to 13.0.times.10.sup.-6 /K at room temperature. If, during the circuit board production process, the working surface of the pressing plate is pressed against the copper foil of the laminate and the system is brought to a curing temperature of, for example, 230.degree. C., the copper with a thermal coefficient of linear expansion of 16.8.times.10.sup.-6 /K expands more than the compression surface of the mold. This difference in expansion behavior of the mold and the coating of the laminate upon heating to the curing temperature of the resin can result in warping and wrinkling of the copper foil and make such printed circuit boards unusable.
To avoid this disadvantage, it has already been attempted to make the pressing plates from a material which has a thermal coefficient of linear expansion which is at least close to that of the coating metal copper. Besides hard bronzes, austenitic steels and similar alloys, whose thermal coefficient of linear expansion is from 16.times.10.sup.-6 /K to 17.times.10.sup.-6 /K, have been used. While wrinkles and warping of the coating foil during curing of the laminate resin due to a temperature increase during molding can be avoided with these mold materials, the service life of such pressing plates is short. This disadvantage is caused by the fact that, due to the low hardness as well as the flow characteristics under local pressure stress of these materials and because of their low wear resistance, dents caused, for example, by foreign bodies, such as grains of dust and the like and/or borders or edges of the laminate, may be formed in the working surface. During compression of the next circuit board, these dents create a bump on the metal coating which is disadvantageous since upon etching of the conductors the Ohmic resistance in the corresponding layer area will differ from that in the rest of the layer.